There is emerging a new hypothesis that soluble high molecular weight (HMW) oligomeric complexes are the neurotoxic species whereas the insoluble inclusion bodies are a protective reservoir of non-toxic aggregates. This laboratory recently discovered that tissue transglutaminase (TTG) crosslinking of expanded polyQ proteins promotes formation of soluble high molecular weight crosslinked (HMW-XL) complexes. These complexes could be important intermediates in the pathogenesis of these disease. TTG was identified as a therapeutic target in these diseases based on several findings including the: 1) identification of expanded polyQ proteins as TTG substrates, 2) detection of epsilon-glutamyl-gamma-lysyl (designated as isopeptide) bonds in Huntington's disease (HD) patients, 3) demonstration of cystamine, a TTG inhibitor, reduced disease severity and 4) TTG knock-out (TTG -/-) mice cross-bred with HD mice had less severe disease phenotype. These data provide strong support that inhibition of TTG activity is beneficial for these diseases. There is controversy, however, regarding cystamine as it is a non-specific TTG inhibitor and associated with drug toxicity. Clearly, a specific TTG inhibitor is needed to treat these diseases and will be useful to define the mechanism of TTG in the pathogenesis of expanded polyQ diseases. We postulate that chemicals that inhibit formation of TTG-dependent soluble HMW-XL complexes could have therapeutic value in these neurodegenerative disorders. The current proposal deals with the high throughput screening and development of potential therapeutic inhibitors to block TTG's ability to form soluble crosslinked neurotoxic intermediates using a unique in vitro solubility assay. The inhibitors will be used to investigate the role of TTG in the pathogenesis of expanded polyQ diseases. Current proposal will lead to new therapy for HD and other neurologic disorders. [unreadable] [unreadable] [unreadable]